1. Field of the Invention
The present invention relates to a projection exposure apparatus used in a process for manufacturing semi-conductor elements, and more particularly, it relates to a projection optical system for a reduction projection exposure apparatus.
2. Related Background Art
In the past, a reduction projection exposure apparatus has been used for manufacturing a semi-conductor element having a micro-pattern, such as LSI, VLSI and the like, and considerable effort has been made to improve for transferring a more minute pattern correctly and stably. For example, the improvements regarding the use of light having shorter wavelength and the increase of NA (numerical aperture) of a projection optical system to expose a wafer have been still continued. Further, in order to stably transfer such minute pattern, it is requested that not only the resolving power of the projection optical system must be excellent but also the contrast of the projected image must be high, and, therefore, the effort to find the optimum exposure condition by examining various illumination conditions for illuminating the reticle has been made. With respect to the illumination condition, a technique in which NA (numerical aperture) of the illumination optical system and NA of the projection optical system are adjusted to obtain a proper balance between the resolving power and the contrast regarding a given pattern by adjusting a so-called .sigma. value corresponding to a ratio of the NA of the illumination optical system to the NA of the projection optical system has already been known, for example, as disclosed in the Japanese Utility Model Laid-Open No. 61-151.
However, in the conventional apparatus disclosed in the above-mentioned Japanese Utility Model Laid-Open No. 61-151, although the illumination optical system is improved in some extent, since the more the NA of the projection optical system is increased the shallower the depth of focus, there arose a problem that, due to the severe or critical focus registration condition for exposing and transferring the minute pattern on the reticle onto the wafer, the pattern could not be correctly transferred to the wafer even when slight variation in focusing occurred. Further, in most cases, since the transfer pattern has not a single cyclic texture or structure, but a plurality of different minute structures, in ordinary projection optical systems, the best focus position tends to be fluctuated in accordance with widths of lines in the minute pattern. Accordingly, the severe adjustment and control had to be performed in the manufacturing process, for example, since the best focus position varies in accordance with the degree of the minuteness of the pattern to be transferred.
Consequently, in the process for manufacturing the VLSI and the like, it was always requested for performing the very critical focus detection and reliable correction of the focus position, thus making the apparatus large-sized, extending the operation time and decreasing the through-put.